1. Field of the Invention
The present invention relates to a technique of calculating the outline of an object from X-ray transmission data obtained by irradiating an X-ray to an object.
2. Description of the Related Art
A technique of irradiating an X-ray to an object, detecting the X-ray having transmitted through the object, reconstructing the obtained X-ray transmission data and thereby creating an image of the inside of the object has been provided. In this image reconstruction, it is possible to obtain a more accurate image if there is information on the outline of an object, such as the size of the object.
Conventionally, information on the outline of an object is calculated based on a scanogram. A scanogram can be obtained by capturing a scanogram of an object in advance. Then, based on the calculated outline of the object, such a scan plan is made that includes the conditions of a full scan of imaging while rotating an X-ray tube and a detector around the object. For example, the radiation dose of the X-ray is set smaller at a thinner portion of the object, whereby the amount of exposure in a full scan can be reduced, and a favorable image can be obtained.
In order to increase the image quality in reconstruction based on the X-ray transmission data obtained from the scan under the above conditions, it is preferred to change parameters used for reconstruction, depending on the size of the outline. A general method for ascertaining the size of a site in reconstruction is a method of extracting the outline of an object from an image having been restructured once. After the outline is extracted by this method, an image is reconstructed again by using parameters reflecting the outline. However, this requires the reconstruction twice. Therefore, a real-time characteristic is lowered. Moreover, it is necessary to reflect, on the second reconstruction, the result of measurement of the outline with an image obtained in the first reconstruction, which results in a complicated process.
Then, in order to solve this problem, as disclosed in Japanese Unexamined Patent Application Publication JP-A 2003-199739, the outline of an object is estimated from a scanogram, namely, 2-dimensional information obtained by scanning in the axial direction of the object as rotation of the X-ray tube around the object is stopped, the parameters are determined depending on the estimated outline size to perform a full scan, and an image is reconstructed based on the X-ray transmission data obtained in the full scan. According to this solution, image reconstruction needs to be performed only once.
However, in a case where a scanogram is used in estimation of the outline of an object, there is a possibility that the position of the object is different between when a scanogram is measured and when a full scan is performed, because the scanogram is obtained prior to the full scan. Therefore, the reconstructed image may be inaccurate when a full scan is performed by using parameters dependent on the outline obtained from the scanogram.